Understanding gravity

In 1919, two expeditions, let by Sir Arthur Eddington set out to observe a total eclipse of the Sun, and perhaps prove or disprove Einstein’s notion of gravity. The parties were able to observe the bending of starlight as it passed the Sun, influenced by our star’s gravity, as predicted by Einstein. Despite some question of their results, their work combined with another observation in Western Australia in 1922 is together seen as proof of Einstein’s idea. Ron Cowen describes how our understanding of gravity has developed, and how it might change with results from future experiments and observations.

Transcript

Robyn Williams: And here once more is the scientist of the century. Every century.

Albert Einstein: A permanent peace cannot be prepared by threats but only by the honest attempt to create mutual trust. One should think that the wish to create a decent form of life on this planet and to avert the danger of unspeakable destruction would tame the passions of responsible men. You cannot rely on that, my young friends. May you succeed in activating the young generation in this sense so that it will strive for a policy of peace on a grand scale. Thus you cannot only defend yourself successfully but you can serve your country and your descendants in a degree as was not given to any previous generation.

Robyn Williams: Einstein on trust and the promise of youthful engagement. But now we turn to Einstein's physics. Do you remember Tamara Davis on ABC television earlier this year on black holes?

Excerpt from Catalyst:

Geoff Bower: So first off, if the ring just isn't there, that would be shocking. All of Einstein's theory tells us that the ring should be perfectly circular. And so if we see it squashed down or if it's imperfect in some sense or if it's not even a ring, if something truly bizarre shows up, then that tells us that something about general relativity breaks down when you get into this extreme regime.

Tamara Davis: It would be pretty surprising to us both, I think, if Einstein's theory proved to not be right. What would you feel if that happened?

Geoff Bower: You know, it would be and amazing thing to discover that the theory of general relativity is incorrect. It's not very likely, but it would be the greatest moment of my scientific life to be able to show that.

Tamara Davis: It's a photograph that I can't wait to see. And even though it might look different to how we imagine, I think it's going to be another one of those images in the story of science that unlocks a whole new era of understanding.

Robyn Williams: Tamara Davis from the University of Queensland presenting a terrific film in the first series of Catalyst, and that's up for a Eureka Prize at the end of this month. And yes, the pictures from the black hole did not trash Einstein. The old man still rules. And Ron Cowen in the States has just published a book on Einstein's century of gravity.

My first question is in many ways a ridiculous one. What is gravity?

Ron Cowen: That's actually a really great question, people are still puzzling what that is. People as long ago as Galileo and more recently Newton thought of it as a force that pulls mass, but what Einstein says is that it is really space-time itself or really the curvature of space-time that is the same thing as gravity, he believed, and observations indicate that it is not a force, it's geometry, it's when a heavy object warps geometry so that there is a local dent in space-time, objects fall towards that dent, not because there's a force but because there is a dent literally in the space-time that objects are forced to follow.

Robyn Williams: What is the century of gravity, when did it start and when does it finish?

Ron Cowen: The century starts May 29, 1919. It was one of the longest solar eclipses of the century, and it's really important because it was an eclipse that people knew about and it was just after the end of World War I where people finally had the equipment together that they were going to observe the Sun during the eclipse and observe the stars around it and see if it was really true, as Einstein said, that the path of starlight would be bent by the Sun's mass. And certainly at that point you couldn't see that unless the brilliant Sun itself was occluded, blocked by the Moon. And so that was the beginning.

It really was also quite poignant that this eclipse, the expedition to Brazil and an island off the west coast of Africa, it was headed by two British teams led by Sir Arthur Eddington, verifying a crazy theory about gravity by a German born scientist, Albert Einstein, who developed it behind enemy lines. People were still talking about the evil Hun. All the violence and death of World War I was fresh in everyone's mind.

Robyn Williams: Okay, the experiment seemed to work, the light was moved by the heavy object, the gravity, but there is some argument as to whether the experiment itself was exact and proved the point, so much so that a number of people have said that it had to be redone actually in Western Australia, something like 1923, and that was the clear and obvious proof that 1919 worked. But 1919, headlines around the world, it's all different, the world has changed. But the two together really were sensational, weren't they.

Ron Cowen: I think that's true, and what some modern astronomers say is that there were a confluence of different types of errors in the 1919 observations that kind of cancelled each other. And so by luck, this is what some modern astronomers say, they really were relatively accurate, no pun intended, and did get the right answer. And there was luck involved because one of the instruments they were relying upon, one of the telescopes, the lens deformed in the heat in Brazil, and luckily a little backup telescope (the little telescope that could, I think of it) really saved the day because in Príncipe, the island off the west coast of Africa where Arthur Eddington and his colleague was, it was cloudy, it was a rain storm that morning, it was touch and go with the clouds, and they had so-so observations, not great ones. But the two observations together in Brazil and west coast of Africa really clinched it. But you're right, I think more observations were needed and were done.

Robyn Williams: And the West Australian ones we've talked about on The Science Show. It was David Blair, the professor of physics at the University of Western Australia who did the talk on that, if you want to look at up. But then before the century we had Newton. How much was Newton's idea adjusted?

Ron Cowen: Well, I will say and it is important to note, Newton had wonderful predictions that explained the motions of the planets, it explained most of the motions of the moons. Isaac Newton does a great job, but one thing that Newton himself objected to is in his theory if you have a heavy object halfway across the Milky Way, thousands of light years away, according to his theory, that object instantaneously pulls another object millions of light years away, immediately. And Einstein, with his special theory of relativity, and the fact that things get communicated by the speed of light and there's nothing faster than the speed of light, he had a problem with that, and Newton himself thought that could be a problem. And in Einstein's theory of gravity, that is taken care of, and there is no instantaneous action, it does take time for a force or the curvature of space-time, if you like, to be communicated.

Robyn Williams: And of course this is still being followed up 100 years later. Andrea Ghez at UCLA, whom you mentioned in the book, looking at the gigantic black hole in the centre of our galaxy, seeing whether Einstein's laws are in fact held up by what…how does the black hole in the centre of the galaxy help her to do that?

Ron Cowen: Well, the black hole is such a concentrated dense mass that the stars right around it are whipping around at nearly the speed of light, and their light is shifted in wavelength as well, stars in their path that appear to be coming towards us are shifted in wavelength towards the blue, they are shifted towards the red when they go away from us. And so there's all kinds of motions that Einstein's theory predicts, and you can track these stars, as Andrea Ghez has done and another team in Germany, and they've done it now for more than 20 years. And the motions of these stars betray the black hole and they betray the properties of the black hole and the properties of space-time that this black hole has so influenced.

Robyn Williams: Do a thought experiment. I have just removed the black hole from the centre of the galaxy, the Milky Way, it's gone. What suddenly differs about where we live, about the galaxy, about what things would do? Would everything be terribly upset?

Ron Cowen: Certainly not immediately. There would be a deficit of mass, but that mass is 4 million solar masses times the heft of the Sun. That is not a hell of a lot at all compared to all the mass in the Milky Way. And if you were far away from a black hole, you aren't affected by it in the first place. You can only be sucked in, so to speak, if you are quite close to the black hole. So if you are far away you would not notice much. Eventually the structure of the centre of the Milky Way would change, but it would be gradual.

Robyn Williams: Okay, let's go back to the first question I asked at the beginning—what is gravity—I'm going to ask it with a little hook as well; what is quantum gravity?

Ron Cowen: Right. Quantum gravity is when you try to understand gravity when you are dealing with the smallest bits of space and time, when you are looking at the atomic scale. And the reason that black holes come into that is that at least, as they say, classically, inside a black hole, space-time and everything else, the very centre is squeezed to a point. Everything is crushed down, actually equations break down, but intrinsically a black hole is dealing not only of course with gravity, which means general relativity, but at the same time with the tiniest of distances. So a black hole is really…you can think of it as the place where quantum and gravity must be married to each other, must be wed. And people had been looking to understand exactly the nature of this marriage, how were they married. Einstein spent the last decades of his life looking for the answer and never found it.

And one interesting thing though, an ironic thing is Einstein always objected to quantum theory. He didn't like the uncertainties, he didn't like the fact that you couldn't know at the same time exactly the position and velocity of a particle. He thought maybe there was something hidden, if we understood the universe, everything would be certain. But it turns out that some of the weirdest, strangest properties of quantum theory, the very ones that Einstein most objected to, may actually point the way to understanding quantum gravity and that marriage.

Two subatomic particles can be correlated in a special way that if you observe the state of one, which…before you observe or measure they were completely uncertain, let's say there's one black object and one white object, but each particle could be half black and half white, that's the way quantum theory is. If you observe one is black, the other one, even if that other particle, the partner now separated and has travelled somehow across the universe, it forces that other partner to be in the other quantum state. Einstein thought this was ridiculous, science fiction, crazy, it's called quantum entanglement. Now some people…it's controversial, some people think the very thing, the very quantum entanglement idea gives rise to the threads of space and time. And since space and time is what Einstein called gravity, it is somehow giving rise to gravity.

Robyn Williams: The old man scores again. The book is Gravity's Century; From Einstein's Eclipse to Images of Black Holes, published by Harvard University Press.

Further Information

Credits

Comments (1)

Rodney Bartlett :

22 Aug 2019 6:03:09pm

This comment was inspired by statements in a video of an astrophysics course presented by Paul Francis & Brian Schmidt (yes, the winner of the Nobel Physics Prize). The following explains analogy of quantum spin to maths’ matrix and how, using that analogy, the statements can be converted into a math origin of electromagnetism & gravitation.

These scientists support the idea of the universe being composed of information/maths:

a) John Wheeler suggested that information is fundamental to the physics of the universe. (John Wheeler: Information, physics, quantum: The search for links) b) Prof Verlinde says gravity is an emergent phenomenon emerging from the changes of fundamental bits of information, stored in the very structure of spacetime. (arxiv.org/abs/1611.02269) c) Cosmologist Max Tegmark hypothesizes that mathematical formulas create reality. (Max Tegmark, Our Mathematical Universe)

Quantum spin is a fundamental property of particles with no counterpart in classical physics. Any picturesque comparison of a spinning electron to a spinning top is a poor aid. My idea is that quantum spin may be analogous to the matrix of mathematics. Matrix multiplication says X multiplied by Y does not always equal Y times X. Think of the matrix as composed of two bar magnets, with the bar magnets being composed of photons & gravitons instead of atoms (the energy & momentum of the photons & gravitons exert a pressure which we call mass when it’s in atoms). Physics says the electromagnetic field fills all space & so do its quantum modes. (Since matter exists in space, let's assume for the moment that the photons also fill all matter). Why photons & gravitons? Because an analogy of gravitational & electromagnetic fields is seen by comparing the Field Equations from General Relativity with Maxwell's Field Equations for electromagnetism.

Now let's return to "Matrix multiplication says X multiplied by Y does not always equal Y times X" and "the matrix composed of bar magnets". XY can represent magnetic North on one bar attracting magnetic South on the other. YX can represent South repelling South. When different magnetic poles (or different combinations of the two letters) are close together, you'll feel an energy difference between them. When quantum spin changes from XY's attraction to YX's repulsion, nano sized bits of the magnets (the photons & gravitons composing them) are repelled or emitted, accounting for electromagnetic & gravitational waves. In reality, the magnets are merely a substitute for the matrix which appears to truly be analogous to quantum spin & also appears to provide a purely mathematical origin for electromagnetic & gravitational waves.